1. Field of the Invention
The present invention relates to an electrostatic latent image developing magenta toner (hereinafter may be simply referred to as “toner”), to an electrostatic latent image developer, to a method for manufacturing an electrostatic latent image developing magenta toner, and to an image forming method, all of which are used in devices that perform a xerographic process, such as a copier, a printer, or a facsimile, and in particular a color copier.
2. Description of the Related Art
Many types of xerographic processes are conventionally known, such as the method disclosed in Japanese Patent Publication No. Sho 42-23910. In a xerographic process, a latent image is electrically formed, by a variety of means, on a photosensitive member composed of a photoconductive material. The latent image is developed by means of a toner. The toner latent image formed on the photosensitive member is transferred onto a receiver sheet composed of a material such as paper, with or without use of an intermediate transfer member, so as to create a toner image on the sheet. The transferred image is fixed by means of heating, pressurization, heating under pressure, solvent evaporation, or other methods. By performing the above-described steps, a fixed image is produced. Before the above steps are repeated, toner which remains on the photosensitive member is cleaned off as necessary by various methods. In recent years, along with technical advancements in the field of xerography, xerographic processes are being employed not only in regular copiers and printers, but also for near-print purposes. In addition to providing high device speed and reliability, xerographic processes are expected to meet increasingly strict demands for high image quality and hue equivalent to those achieved by a printing press. In particular, red and magenta are important colors, in view of their strong influence on improvement of the impression of an image.
Conventionally, toner is fabricated by means of a kneading-and-grinding method. According to the kneading-and-grinding method, a binding resin is melted and kneaded with additives such as a colorant and a release agent, and subsequently grinded. In a toner obtained by grinding, the colorant and the release agent may become exposed on the toner surface, possibly exerting negative influences on the charging property and life of the toner. Further, when a release agent having a lower melting point is employed to achieve a desired low-temperature fixation property, and an increased amount of release agent is added in order to achieve the preferred oil-less fixation, the release agent may melt out of the toner during the melting and kneading step. Moreover, system viscosity may lower, possibly causing maldistribution of additives within the toner. As such, the kneading-and-grinding method may negatively influence not only the charging property and life of a toner, but also the attained image quality, including color and density. For these reasons, instead of the melting-kneading-grinding method, wet fabrication methods have often been used to fabricate toner in recent years. For example, Japanese Patent Laid-Open Publication Nos. Sho 63-282749 and Hei 6-250439 describe an emulsion polymerization aggregation method. According to this method, resin particles are prepared by emulsion polymerization. A colorant-dispersed liquid having a colorant dispersed within an aqueous medium is also prepared. In accordance with needs, a release-agent-dispersed liquid having a release agent dispersed within an aqueous medium is further prepared. These prepared materials are mixed, and aggregate particles are formed in the mixture by heating or other methods. Subsequently, the aggregate particles are fused by heating to obtain the resulting toner.
Because color images have been used widely and often in recent years, image preservability is another point that is considered important. Conventionally, thermoplastic resin is used to fabricate electrostatic latent image developing toner (hereinafter may be simply referred to as “toner”). In order to simultaneously attain both low-energy fixation and powder blocking property, control for optimization of rheology and glass transition point (hereinafter referred to as “Tg”) of resins used for toner is carried out, as described in Japanese Patent Publication No. Hei 2-37586, Japanese Patent Laid-Open Publication No. Hei 1-225967, and Japanese Patent Laid-Open Publication No. Hei 2-235069. In electrostatic latent image developing toners which may be used to print near-print documents, resins having lower Tg are generally selected and employed in order to achieve high-speed fixation. However, an image formed from a toner fabricated on the basis of such techniques is disadvantageous in that, when the image is subjected to heating at a temperature around or higher than Tg, the resin component in the image portion may melt, resulting in adhesion of toner onto the backside of a sheet laid on top or onto other printed sheets. This would cause the image to be defective. Moreover, because near-print documents are often produced by double-sided printing, image portions formed on separate sheets are more likely to be placed in contact with one another as compared to the case where printing is performed on one side only, thereby more frequently causing the above-described image defects (image defects caused in this manner are hereinafter collectively referred to as “image offsets”). In general, whereas black-and-white images mainly comprise text, color images often include numerous graphics. Accordingly, in a color image, the proportion of toner coverage area with respect to sheet area tends to be larger, which is another factor that may cause more frequent image offsets. Image offsets would obviously occur at temperatures higher than Tg, but even at temperatures lower than Tg, image offsets may occur when an image is subjected to high pressure over a long period of time. In color printing, toner preservability may depend on the color of the toner. More specifically, a toner of a specific color may have lower preservability as compared with toners of other colors. Typically, color printing basically employs four toners; black toner, and three color toners consisting of cyan, yellow, and magenta. A cyan toner generally has favorable preservability, which may be influenced by the types and addition amounts of pigments in the toner. However, details as to the basis of high preservability of cyan toners have yet to be clarified. Yellow is a color which is not very noticeable even when image offsets occur to some extent. Therefore, improving preservability of magenta toners is important.
In magenta toners, quinacridone pigments are mainly used, as described in, for example, Japanese Patent Laid-Open Publication Nos. Hei 1-154161 and Hei 2-32365. Further, naphthol pigments may be employed, as disclosed in Japanese Patent Laid-Open Publication Nos. Hei 5-19536, Hei 11-272014, 2001-166541, and 2001-249498. Japanese Patent Laid-Open Publication Nos. Hei 4-226477, Hei 5-142867, 2000-199982, 2002-156795, and 2003-215847 describe using quinacridone pigments and naphthol pigments in combination. Although the above-listed documents describe magenta toners which are enhanced as compared with those of the prior art (in particular, use of quinacridone and naphthol pigments in combination provides favorable magenta toners) with respect to image quality, none of the documents make any reference to image preservability.
The present invention is directed to solving the above-described problem; that is, to improve image preservability of magenta toners to a level equivalent to those of other colors while employing quinacridone pigments and naphthol pigments which have favorable coloring property, developing property, transferability, charging property, and fixation property.